In recent years, an environmentally-friendly vehicle technology has becoming a very important technology in the automotive industry. Vehicle manufacturers have been focusing on development of environmentally-friendly vehicles, such as electric vehicles (EV), hybrid electric vehicles (HEV), and fuel cell electric vehicles (FCEV), to satisfy the environment and fuel consumption regulations.
These eco-friendly vehicles have technical restrictions in terms of weight and cost, and thus, the vehicle manufacturers keep observation on the hybrid electric vehicles for the exhaust gas regulations and improving fuel consumption performance.
A hybrid electric vehicle is a vehicle using more than two power sources. A gasoline or diesel internal combustion engine using fossil fuel and a motor/generator driven by electrical energy are the main power source for the hybrid electric vehicle.
The hybrid electric vehicle uses the motor/generator having relatively low-speed torque characteristics as a main power source at a low-speed and uses the engine having relatively high-speed torque characteristics as a main power source at a high-speed.
Since the hybrid electric vehicle stops the engine operation using the fossil fuel and uses the motor/generator at a low-speed region, fuel consumption may be improved and exhaust gas may be reduced.
However, a power transmission system of the hybrid electric vehicle has different durability, power delivery efficiency, and the size that depends on connections of planetary gear sets, the motor/generators, and a torque delivery apparatus. Therefore, designs for the connection structure of the power transmission system of the hybrid electric vehicle are also under continuous development to achieve robust and compact power transmission system having no power loss.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention, and therefore, it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.